This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce images of acquired harmonic signals.
The use of ultrasonic signals which are harmonically related to transmitted ultrasound signals for ultrasonic diagnostic imaging is described in U.S. Pat. No. 5,833,613 (Averkiou et al.) and U.S. Pat. No. 5,879,303 (Averkiou et al.) The ""613 patent describes a number of techniques for imaging with harmonic contrast agents. Ultrasonic contrast agents are comprised of tiny encapsulated microbubbles which, when struck by a transmitted ultrasound wave, will exhibit nonlinear resonance, including resonance at harmonic frequencies of the transmitted wave frequency. This nonlinear resonance will return an echo signal containing the harmonic frequencies in addition to components at the fundamental (transmit) frequency. While the harmonic components are not as great in intensity as the fundamental components, they are nonetheless of relatively significant intensity and can be readily detected and discriminated to provide segmented contrast signal information.
The ""303 patent describes another form of ultrasonic harmonic imaging known as tissue harmonic imaging. Tissue harmonic imaging relies upon the distortion of a transmitted wave which occurs as the wave passes through the tissue of the body. This distortion gives rise to nonlinear signal components including those at harmonics of the fundamental transmit frequency. The tissue harmonic signal components are of a lesser relative intensity as compared to contrast harmonic signal components, but may nonetheless be readily detected and used to form ultrasonic images. As explained in the ""303 patent, tissue harmonic imaging prevents the occurrence of nearfield and other image artifacts which are common to fundamental signal images.
In both contrast and tissue harmonic imaging it is necessary to distinguish or separate the harmonic signal components from the accompanying fundamental frequency components. Efforts to effect this separation have focused on filtering techniques such as bandpass filtering, and signal processing techniques such as the multiple echo technique known as pulse inversion. However, all of these techniques can be impeded by limitations in signal acquisition, the apparatus and processing used to initially acquire the harmonic signals. One such limitation is inherent in the use of digital beamforming, which is in widespread use in virtually all of today""s premium ultrasound systems. The initial step in digital beamforming is the digital sampling of received echo signals by analog to digital conversion. Since the harmonic components are many dB down from fundamental signal component amplitudes, particularly in the case of tissue harmonic signals, much of the dynamic range of the digital echo samples will be occupied by fundamental signal information. The fundamental signal components can even be of a strength which overwhelms or saturates the analog to digital converter, thereby rendering the harmonic components undetectable in subsequent harmonic discrimination or separation processing. Moreover, saturation will generate odd harmonics which can fall at even harmonic frequencies of interest, which is particularly troublesome in broadband imaging systems. It would be desirable to prevent such saturation and inability to detect the harmonic signal components. It would further be desirable to use a significant portion of the dynamic range of the analog to digital converter for conversion of the harmonic signal components.
In accordance with the principles of the present invention, the fundamental frequency components of an echo signal are selectively attenuated prior to digitization of the echo signal. In one embodiment one or more of a plurality of filter circuits are selectively switched into use prior to the analog to digital converter to attenuate fundamental frequency signal components. In another embodiment the characteristic of a programmable filter circuit is varied during receipt of echo signals in correspondence with depth dependent frequency attenuation. In yet another embodiment a plurality of filter circuits are sequentially switched into use to track the depth dependent changes of the echo signal.